The invention relates generally to methods and systems for the directional drilling of wells, particularly wells for the production of petroleum products. More specifically, it relates to steerable systems run on coiled tubing.
It is known that when drilling oil and gas wells for the exploration and production of hydrocarbons, it is often necessary to deviate the well off vertical and in a particular direction. This is called directional drilling. Directional drilling is used for increasing the drainage of a particular well by, for example, forming deviated branch bores from a primary borehole. Also it is useful in the marine environment, wherein a single offshore production platform can reach several hydrocarbon reservoirs, thanks to several deviated wells that spread out in any direction from the production platform.
Directional drilling systems usually fall within two categories: push-the-bit and point-the-bit systems, classified by their mode of operation. Push-the-bit systems operate by applying pressure to the side walls of the formation containing the well. Point-the-bit systems aim the drill bit to the desired direction, thereby causing deviation of the wellbore as the bit drills the well's bottom.
Push-the-bit systems are known and are described, for example, in U.S. Pat. No. 6,206,108 issued to MacDonald et al. on Mar. 27, 2001, and International patent application no. PCT/GB00/00822 published on Sep. 28, 2000 by Weatherford/Lamb, Inc. These references describe steerable drilling systems that have a plurality of adjustable or expandable ribs or pads located around the corresponding tool collar. The drilling direction can be controlled by applying pressure on the well's sidewalls through the selective extension or retraction of the individual ribs or pads.
Point-the-bit systems are usually based on the principle that when two oppositely rotating shafts are united by a joint and form an angle different than zero, the second shaft will not orbit around the central rotational axis of the first shaft, provided the two rates of rotation of both shafts are equal.
Various point-the-bit techniques have been developed which incorporate a method of achieving directional control by offsetting or pointing the bit in the desired direction as the tool rotates. One such point-the-bit technique is outlined in U.S. Pat. No. 6,092,610 issued to Kosmala et al. on Jul. 25, 2000, the entire contents of which are hereby incorporated by reference. This patent describes an actively controlled rotary steerable drilling system for directional drilling of wells having a tool collar rotated by a drill string during well drilling. The bit shaft is supported by a universal joint within the collar and rotatably driven by the collar. To achieve controlled steering of the rotating drill bit, orientation of the bit shaft relative to the tool collar is sensed and the bit shaft is maintained geostationary and selectively axially inclined relative to the tool collar. This position is maintained during drill string rotation by rotating it about the universal joint via an offsetting mandrel that is rotated counter to collar rotation and at the same frequency of rotation. An electric motor provides rotation to the offsetting mandrel with respect to the tool collar and is servo-controlled by signal input from position sensing elements. When necessary, a brake is used to maintain the offsetting mandrel and the bit shaft axis geostationary. Alternatively, a turbine is connected to the offsetting mandrel to provide rotation to the offsetting mandrel with respect to the tool collar and a brake is used to servo-control the turbine by signal input from position sensors.
Current rotary steerable systems are run on drill string and thus inherit the operational limitations associated with the drill string. An attempt has been made to combine a rotary steerable system with coiled tubing as described in U.S. Pat. No. 7,028,789. This reference discloses an integrated motor and steering system for coiled tubing drilling. However, as will be discussed below, the apparatus described in the U.S. Pat. No. 7,028,789 has several inherent disadvantages overcome by the teachings of the present invention.